Method of forming bumps

ABSTRACT

In the conventional bump forming method that can be applied to a semiconductor device in which a large number of bumps are required to form, there are various limitations to the material of which the bumps are made, to enough cubic volume of bumps and to small scattering of the bump height. According to the invention, solder balls and a tool having a large number of through-holes are used, and under the condition that the through-holes of the tool are aligned with the pads of the semiconductor device, the solder balls are charged into the through-holes, pressed to be fixed on the pads, and then reflowed to form bumps.

This application is a Continuation-in-Part application of Ser. No.09/315,818, now U.S. Pat. No. 6,213,386 filed May 21, 1999, the contentsof which are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a method of forming bumps aselectrical, mechanical and thermal connections or contacts on asemiconductor device.

There are many examples as to the method of forming bumps on asemiconductor device. For example, metal is deposited by plating to formbumps on the pads of a semiconductor device (plating method); a solderpaste is printed on the pads of a semiconductor device, heated so thatsolder particles within the paste are melted, and thereafter solidifiedto form bumps on the pads (printing method); and one end of gold wire isbonded to a pad, and then the wire is cut so that the bonded part ofwire is left as a bump (wire bonding method). In addition, as disclosedin U.S. Pat. No. 5,284,287, solder balls are sucked in the cavities of apick-up tool by vacuum, placed on the pads of a semiconductor device andheated to melt, and solidified to form bumps (conventional type solderball method).

These conventional methods, however, have the following drawbacks. Ingeneral, the larger the cubic volume of the bumps, the longer the lifeof the connection between the semiconductor device and an electroniccircuit board through the bumps can be extended. In the plating methodand printing method, however, it is difficult in principle to form bumpsof enough cubic volume. Moreover, since the heights of the bumpsscatter, all the bumps cannot be properly connected between thesemiconductor device and, the electronic circuit board. In the wirebonding method, the material of the wire is limited to only a particularone such as gold. Also, since bumps are produced one by one, it takes avery long time to produce many pads as for a semiconductor device havingtens of thousands of pads.

In the conventional type solder ball method, the scattering of the bumpheight is small, and bumps of enough volume can be produced, but thepick-up tool for use in sucking balls by vacuum to hold is complex instructure and requires a delicate perforating technique for very fineholes or cavities when it is produced. Since this tool becomes expensivein proportion to the number of holes required, the cost of forming bumpsincreases when the tool is used for a semiconductor device that needs alarge number of bumps.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method of forming bumps ofenough cubic volume which have small scattering of bump height and nolimitation to material to be selected, and which method can be used forthe bumps of a semiconductor device that needs a large number of bumps,or is able to produce bumps fast and at low cost, thus providinglow-cost semiconductor devices to the market.

In order to achieve the above object, the present invention has executedthe following means. First, conductive spheres such as solder balls arefundamentally used in order to provide bumps of enough cubic volume withsmall scattering of bump height and with no limitation to material to beselected. In addition, to actualize the high-speed, low-cost productionof bumps that can be applied to the production of bumps in asemiconductor device that needs a large number of bumps, low-cost toolssuch as a printing metal stencil and brush that are moved in parallelare used to place a large number of solder balls on a semiconductordevice at a time, which are then pushed against the pads of thesemiconductor device by a pressing tool so that the bumps to be formedcan be prevented from being defective, and thereafter the solder ballsare heated to form bumps. Moreover, it is checked if the produced bumpsare excessive or insufficient, and if necessary, re-trying operation isperformed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is part of a flow diagram of bump formation according to theinvention.

FIG. 2 is another part of a flow diagram of bump formation according tothe invention.

FIG. 3 is the last part of a flow diagram of bump formation according tothe invention.

FIG. 4 is a diagram of an example of a semiconductor device and amagnified part thereof.

FIG. 5 is a diagram of a tool for use in this invention.

FIGS. 6A and 6B are diagrams of the semiconductor device with fluxprovided.

FIG. 7 is a diagram of the tool and the semiconductor device alignedwith each other, and which have a gap set between the stencil of thetool and the semiconductor device by a rectangular spacer.

FIG. 8 is a diagram of the tool and the semiconductor device alignedwith each other, and which have a gap set between the stencil of thetool and the semiconductor device by a wire-shaped spacer.

FIG. 9 is a diagram of the tool and the semiconductor device alignedwith each other, and which have a gap set between the stencil of thetool and the semiconductor device by a projection provided on thestencil surface.

FIG. 10 is a diagram showing an example in which a squeegee is used fora charging process.

FIG. 11 is a diagram showing another example in which air flow is usedfor the charging process.

FIG. 12 is a diagram showing still another example in which vibration isused for the charging process.

FIG. 13 is a diagram showing a still further example in which the tooland the semiconductor device are tilted for the charging process.

FIG. 14 is a diagram showing an example in which a pressing tool withprojections is used when pressing.

FIG. 15 is a diagram showing another example in which a pressing toolwith recesses is used when pressing.

FIG. 16 is a diagram showing still another example in which a pressingtool with an elastic member provided on the pressing surface is usedwhen pressing.

FIG. 17 is a cross-sectional diagram of the through-holes of the toolwith their diameter changed in the direction of their center axis.

FIG. 18 is a diagram explaining in detail the embodiment that a squeegeeis applied in a charging process.

DETAILED DESCRIPTION OF THE EMBODIMENTS

An embodiment of the invention will be described with reference to FIGS.1 to 16. In those figures, like elements are identified by the samereference numerals, and will not be repeatedly described. FIGS. 1, 2 and3 show a fundamental flow of the bump formation according to theinvention. The bump formation flow according to the invention basicallyincludes an alignment process, a ball charging process and a heatingprocess as illustrated in FIGS. 1, 2 and 3. These processes willhereinafter be described in order, with reference to other figures, ifnecessary. Referring to FIGS. 1, 2 and 3, there are shown asemiconductor device 1 on which bumps are to be formed, a tool 2,conductive spheres 3, completed bumps 4, through-holes 6 of the tool 2that will be described later, a brush 7, and a pressing tool 9. Inaddition, there are shown a stencil 2 a of the tool 2, and a frame 2 bof the tool 2. Although the semiconductor device 1 on which bumps can beformed according to the invention may be of different types such as awafer not protected, a wafer protected with resin, a cut-away part ofthe wafer and a packaged cut-away part of the wafer, a wafer-shapedsemiconductor device is used as an example of the semiconductor device1. This is because the effect of the invention is generally great onforming bumps of a wafer-type semiconductor device that needs a largenumber of bumps. FIG. 4 shows the semiconductor device 1 on which bumpsare formed according to the embodiment of the invention, and a magnifiedpart of the device. The semiconductor device 1 has a large number ofpads 5 on which bumps are to be formed, for example, at most tens ofthousands of pads 5. The tool 2 is formed by the frame 2 b and thestencil 2 a as illustrated by the cross-sectional view of FIG. 5. Thestencil 2 a has through-holes 6 formed to oppose to the pads 5 of thesemiconductor device 1. This stencil 2 a can be made of a material ofmetal or resin. The diameters of the through-holes 6 are in the rangefrom that of the electrically conductive spheres 3 to less than twicethat of the spheres 3 that are used in the charging process. Thethrough-holes 6 of this tool 2 can be formed by machining using a drillor by a chemical process such as etching or electroforming. If anappropriate one of those processes is selected according to the numberof through-holes 6, the through-holes can be formed in the tool 2 at arelatively low cost.

The bump formation flow is as follows. First, an adhesive supply processis executed to supply an adhesive on the pads 5 of the semiconductordevice 1. This adhesive may be flux, solder paste or conductiveadhesive. The adhesive such as flux is supplied by printing with astencil or by spin coating using centrifugal force. FIGS. 6A and 6B arecross-sectional diagrams of part of the semiconductor device 1 with anadhesive 10 provided on the pads 5 of the semiconductor device 1. FIG.6A shows the adhesive 10 coated over the entire surface of thesemiconductor device 1 including the pads 5, and FIG. 6B shows theadhesive 10 selectively coated only on the pads of the device 1.

Next, the alignment process is executed to align the pads 5 of thesemiconductor device 1 with the through-holes 6 of the tool 2. Thealignment is performed manually by usual recognition as in the stencilprinter or automatically by using an automatic stage and imageprocessing that pays attention to the features of a video image pickedup by a CCD camera. FIG. 7 is a magnified view of the completedalignment between the device 1 and the tool 2. In FIG. 7, there is showna rectangular spacer 8 a that is set between the device 1 and the tool 2in order to fix the gap therebetween. FIG. 8 shows an example of using awire-shaped spacer 8 b. FIG. 9 shows an example of using a spacer 8 cthat is integrally formed with the tool 2. These spacers can prevent thethrough-holes 6 from being contaminated with the adhesive 10 such asflux that is coated on the pads 5 of the semiconductor device 1. Inother words, if the height of the spacer is selected to be larger thanthat of the supplied adhesive 10 such as flux, there is no risk that thetool 2 is made in contact with the adhesive such as flux.

In the ball charging process, the conductive spheres 3 of which thenumber is larger than that of the bumps being formed are supplied on thetool 2. The supplied conductive spheres 3 are dropped down into thethrough-holes 6 of the tool 2 by the translational motion of the brush 7shown in FIG. 2, or the through-holes 6 are charged with the conductivespheres 3. In addition, the excessive conductive spheres 3 on the tool 2are removed from the regions in which the through-holes 6 occupy on thetool 2 by the translation of the brush 7. In order that the number ofspheres 3 charged into one through-hole 6 of the tool 2 is limited toone, the dimension h shown in FIG. 7 is required to be less than 1.5times as large as the diameter of the conductive sphere 3. The chargingof conductive spheres 3 into the though-holes 6 of the tool 2 can beperformed not only by using the brush 7, but also by using thetranslation of a squeegee 11 (e.g., in the form of a spatula) shown inFIG. 10, using an air flow 13 from an air nozzle 12 shown in FIG. 11,the vibration shown in FIG. 12 or the tilting of the tool 2 and device 1shown in FIG. 13. Also, the excessive conductive spheres 3 can beremoved from around the through-holes 6 by those means. Thereafter, ifnecessary, tests are made to examine if the through-holes 6 have beencompletely charged with the spheres, and if the excessive spheres 3 havebeen completely removed from around the through-holes 6, thus makingsure of the processes. The tests can be performed by the usualrecognition of the operator or by the automatic recognition using thecombination of the electric CCD and image processor. If the chargingoperation and the excessive sphere removal are found not to be complete,the charging process is repeated, the spheres 3 are added, or theexcessive conductive spheres 3 are directly removed by workers, thussolving those problems.

FIG. 18 shows in detail a ball charging process using a squeegee. InFIG. 18, the pads 5 and the adhesive 10 are depicted thicker than thepractical proportion for identifying distinctively in FIG. 18. Here, aspacer 8 c is formed with a tool in one body, and a height of a surfaceof the tool 2 (that is, the upper surface 2 e of the tool) is kept atleast 0.8 times the diameter of the conductive sphere, and, e.g.,smaller than the diameter of the conductive sphere, for example, 0.9times of the diameter of the conductive sphere, from a surface of thesemiconductor device 1. A gap between the tool 2 and the semiconductordevice 1 (that is, a gap between the semiconductor device and a bottomsurface 2 f of the tool 2) is kept at most 0.5 times the diameter of theconductive sphere, e.g., 0.4 times of the diameter of the conductivesphere 3. For such a configuration, the succeeding pressing operationbecomes easy, and it eliminates a problem that the bottom (under)surface 2 f of the tool becomes dirty by flux. Further, since thediameter of the through-hole 6 of the tool 2 is 1.2 times of thediameter of the conductive sphere 3, the conductive sphere is filledinto the through-hole 6 easily, and the conductive spheres equal to ormore than two cannot be filed into the through-hole 6 simultaneously. Inaddition, the height of the under surface 11 c of the squeegee 11 iskept at most 0.5 times the diameter of the conductive sphere 3, e.g.,0.3 times of the diameter of the conductive sphere 3, from the uppersurface 2 e of the tool 2 by the spacer 11 a disposed remotely in thedirection of the sheet of FIG. 18, so that the surface of the squeegee11 facing with the conductive sphere 3 is kept almost perpendicular tothe surface of the semiconductor device 1. Therefore, the conductivesphere 3 a already filled in exists in the surface of the semiconductordevice 1 stably without contacting with the squeegee 11. The conductivesphere 3 b is filled in before the squeegee 11 advances in the directionof arrow 11 e; and, before the squeegee 11 advances, the excessiveconductive sphere 3 c is disposed on the gap between the tool 2 and thealready positioned conductive sphere 3 b. The squeegee 11, as it movesin the direction of arrow 11 e, presses against the excessive conductivesphere 3 c; the direction of the pressing (compression) force is along ahorizontal plane (e.g., parallel to surface 2 e of tool 2), so that theexcessive conductive sphere 3 c can be eliminated effectively. Here,since the contact angle of the excessive conductive sphere 3 c and theconductive sphere 3 b already filled in through-hole 6 is a almosthorizontal plane, the force necessary for the squeegee 11 to eliminateis small enough. Namely, since both the excessive conductive sphere 3 cand the conductive sphere 3 b already filled in through-hole 6 are notsubjected to the excessive stress, the deformations of the pressingtrace, the abrasion and so on are not sustained. Further, the excessiveconductive sphere 3 c is filled successively into the through-holes 6not filled yet with the advance of the squeegee 11, similarly to theconductive spheres 3 d disposed on the tool. Eventually, the conductivespheres 3 which are not used are accumulated at the edge of the tool 2,so that the conductive spheres are retrieved for the next usage. Here,if a pair of squeegees 11 is provided, the conductive spheres not usedcan be moved adversely by the other squeegee not used, so that aplurality of filling processes can be performed successively. Further,there exist the advantages of reducing the dissipation of the conductivespheres during the charging process and of retrieving the conductivespheres not used exactly.

After the completion of the charging of conductive spheres into thethrough-holes 6 and the removal of excessive conductive spheres 3 fromaround the through-holes 6, the pressing tool 9 is moved up and down topress the conductive spheres 3 in the through-holes 6 against thesemiconductor device 1. After the pressure, the tool 2 is lifted awayfrom the semiconductor device 1. In this case, since the conductivespheres 3 stuffed in the through-holes 6 are pressed down, the spheres 3are closely attached to the pads 5 and thus securely fixed on the pads 5by the adhesion of the adhesive 10. Therefore, when the tool 2 is pulledup from the semiconductor device 1, the conductive spheres 3 are left onthe semiconductor device 1, and they are never moved up with the tool 2due to the positional deviation or sticking to the inner walls of thethrough-holes 6. In some case, after the tool 2 is pulled up, theconductive spheres 3 on the pads 5 may be again pressed by the pressingtool 9 in order to increase the effectiveness. By the second pressing,the displacement of spheres 3 that may be caused by the subsequentconveyance and heating process can be suppressed to the minimum. Thepressing tool 9 may have not only simply a flat plate for pressing, butalso projections shown in FIG. 14, recesses shown in FIG. 15 or anelastic body shown in FIG. 16. The pressing tool 9 having projectionsfor use in pressing before the removal of the tool 2 has an effect ofenabling the thickness of the stencil of the tool 2 to be made largerthan the diameter of the conductive sphere as shown in FIG. 14. Thus,since the rigidity-of the tool 2 can be enhanced, the life of the toolcan be extended. By using the pressing tool 9 having recesses on thepressing side, it is possible to precisely position the conductivespheres 3 on the pads 5 of the semiconductor device 1. In addition, byusing the pressing tool 9 having an elastic body on the pressing side,it is possible to press all the conductive spheres irrespective of thedimensional allowances. If an appropriate method of forming thethrough-holes 6, of which the diameter was described previously, isdeveloped to be able to change the diameter of the through-holes 6 inthe thickness direction of the stencil 2 a of tool 2 as shown in FIG.17, the conductive spheres 3 can be easily stuffed into thethrough-holes 6 and placed on the pads 5 of the device 1 with highprecision. In this case, it is desired that on the side where theconductive spheres 3 are supplied, the diameter of the through-hole 6 isequal to or larger than the diameter of the conductive sphere 3 and lessthan twice of the diameter of the conductive sphere 3 (e.g., less than1.3 times the diameter of the conductive sphere 3), and on the side ofthe pad 5, the diameter of the conductive sphere is near to the diameterof the conductive sphere 3 and larger than the diameter of theconductive sphere 3.

In the following heating step, the semiconductor device 1 with theconductive spheres 3 mounted on the pads is placed in a heating furnacesuch as a commercially available reflow furnace. Thus, the conductivespheres 3 can be changed into the bumps 4 connected to the pads of thedevice 1. Thereafter, if necessary, the semiconductor device 1 isrinsed, and cut into necessary sizes, thus the device 1 with the bumps 4being completed.

In this embodiment, tens of thousands of bumps can be formed at a timeon the pads 5 of the semiconductor device 1, thus the mass productivityof bumps being remarkably improved.

According to the invention, a large number of bumps can be formed at atime on the pads of the semiconductor device. In addition, the tool andso on for the production of bumps can be used at low cost, and thedevice structure can be made simple. Moreover, since conductive spheressuch as solder balls can be used as the bump material, bumps ofdifferent constituents can be formed. The bridging and ball vanishingproblems sometimes caused when conductive spheres such as solder ballsare used can be solved by providing the process for pressing theconductive spheres against the pads.

Many different embodiments of the present invention may be constructedwithout departing from the spirit and scope of the invention. It shouldbe understood that the present invention is not limited to the specificembodiments described in this specification. To the contrary, thepresent invention is intended to cover various modifications andequivalent arrangements included within the spirit and scope of theclaims.

What is claimed is:
 1. A method of forming bumps in which conductivespheres are used to form bumps on a semiconductor device, comprising thesteps of: aligning a tool having through-holes with said semiconductordevice so that said through-holes can be opposed to the places wheresaid bumps are to be formed; supplying said conductive spheres on saidtool, charging said conductive spheres into said through-holes of saidtool and removing said tool from said semiconductor device as a ballcharging process; and heating said conductive spheres and saidsemiconductor device, wherein diameters of said through-holes of saidtool on a surface that said conductive spheres are filled into are equalto or larger than diameters of said conductive spheres and smaller than1.3 times of said diameters of said conductive spheres, and diameters ofsaid through-holes of said tool on a surface facing with saidsemiconductor device is equal to or larger than diameters of saidconductive spheres.
 2. A method of forming bumps in which conductivespheres are used to form bumps on a semiconductor device, comprising thesteps of: aligning a tool having through-holes with said semiconductordevice so that said through-holes can be opposed to the places wheresaid bumps are to be formed; supplying said conductive spheres on saidtool, charging said conductive spheres into said through-holes of saidtool and removing said tool from said semiconductor device as a ballcharging process; and heating said conductive spheres and saidsemiconductor device, wherein a height of said through-holes of saidtool from a surface of said semiconductor device is at least 0.8 times adiameter of said conductive spheres and smaller than said diameter ofsaid conductive spheres, and a distance between a bottom surface of saidthrough-holes and said surface of said semiconductor device is equal toor smaller than 0.5 times the diameter of said conductive spheres.
 3. Amethod of forming bumps in which conductive spheres are used to formbumps on a semiconductor device, comprising the steps of: aligning atool having through-holes with said semiconductor device so that saidthrough-holes can be opposed to the places where said bumps are to beformed; supplying said conductive spheres on said tool, charging saidconductive spheres into said through-holes of said tool and removingsaid tool from said semiconductor device as a ball charging process, thecharging of the conductive spheres into said through-holes includingmoving a squeegee along the tool; and heating said conductive spheresand said semiconductor device, wherein a squeegee gap is set between anupper surface of the tool and the squeegee, such that a bottom surfaceof the squeegee is at a height above the tool wherein the squeegee doesnot contact with said conductive spheres filled in said through-holes,and said squeegee gap is equal to or smaller than 0.5 times of adiameter of said conductive spheres disposed on said tool in translationmotion of the squeegee.